1) Field of the Invention
This invention relates generally to a fluid dispensing system/tool, more particularly to fluid dispensing systems for semiconductor manufacturing equipment and particularly to a fluid supply system for dispensing fluid onto semiconductor wafers and more particularly to a sensor monitor for monitoring bubbles and uneven fluid flow in photoresist, etchant or spin-on-glass fluid in a semiconductor manufacturing tool/fluid dispensing system.
2) Description of the Prior Art
Three of the most important operations in semiconductor manufacturing are spin-on-glass coating, photoresist coating and etching steps. First, Spin-on-glass is used as a planarization layer to planarize the uneven surface of a wafer. A spin-on-glass fluid is evenly dripped or sprayed onto a spinning wafer. The spin-on-glass fluid must be applied evenly onto the wafer so that the layer evenly fills the valleys in the surface. The fluid must flow evenly and not contain any bubbles. Bubbles in the spin-on-glass fluid can create defects in the spin-on-glass layer. For example, a bubble in an SOG layer can be etched through in a subsequent etch back step. The etch can expose underlying metal layers and other structures. This can lead to shorts with overlying conductive layers or to disruption of overlying layer structures from falling into "Bubble holes".
Second, resist patterning techniques employed in the semiconductor lithographic process fundamental to integrated circuit manufacturing usually rely on a fluid dissolution step to remove photoresist polymer either made more soluble or left less resistant to dissolution by selective exposure to some type of photon irradiation or particle bombardment. The photoresist is etched to form a photoresist pattern. It is critically important to control this photoresist pattern. Uneven resist flow can lead to uneven photoresist layers and incomplete patterns. Moreover, bubbles in photoresist fluid can also cause photo pattern defects. These defects will cause wafers to be scrapped.
A third important process is the etch step. Etch chemicals (e.g., BOE, buffered HF) must be applied to the wafer to etch the photoresist and the exposed semiconductor layers (e.g., oxide). It is important for the etch chemical to be applied without any bubbles so that the layers are etched evenly.
In these three operations, (i.e., photoresist coating, SOG coating, and etching), a fluid must be applied to a semiconductor wafer in a very controlled even manner so that the photoresist, spin-on-glass and etched layers do not contain any defects. Presently, as shown in FIG. 1, a fluid level sensor 110 is used to ensure that the fluid 120 level in a fluid supply tank 100 is maintained at the proper level. The sensor 110 is connected to an amplifier 120 and to an alarm 130. When the fluid level gets too low the alarm sounds. This system stops the process before a lot of wafers are ruined because of low fluid levels in the tank. However, even with this system some wafers get ruined before the alarm sounds. Alternatively, a lot of good fluid is thrown out if the alarm sounds too soon when sufficient fluid remains in the tank.
However, other problems, such as bubbles and uneven flow, impact fluid flow and cause defects in semiconductor devices on the wafers. These problems are not always related to low fluid levels and are not solved by the tank level sensor of the prior art.
We have found that a major problem in applying fluids to semiconductor wafers is gas bubbles in the fluids. These gas bubbles create defects in the layers over the wafer thus reducing yields. Presently, the gas bubble problem is found only after the wafers are tested and defects found on the wafers. By this time many wafers have been processed and many wafers have the defects.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering U.S. Pat. No. 4,881,487(Moore) which shows a fluid level sensing method and apparatus used in a photoresist tool. A bubble is introduced into the fluid and the bubble is used to detect the photoresist level. U.S. Pat. No. 4,646,796(Krause) shows an apparatus for detecting the level of a liquid in a container filling machine using an infrared photo detector. U.S. Pat. No. 5,493,922(Ramey et al.) shows a liquid level sensing probe and control circuit. U.S. Pat. No. 4,857,750(Millis et al.) shows a sensor for determining the photoresist developer strength.
Yet, there is still a need to develop a method and device for preventing bubbles in SOG flow and chemical fluid flow and for forming a more uniform SOG or photoresist coating on a wafer.